For anyone interested, I captured this accident in my ATCSmon program (I monitor CSX W&A, which also includes the southern part of the Etowah sub, where this occurred) ...
Anyways, on to the show ... on these screencaps, the top line is the south end of the Etowah Subdivision (labeled "C"), the bottom line is the W&A Subdivision (labeled "WA").
White indicates an empty track block, Green indicates a track block that has been cleared for movement, and Red indicates an occupied track block.
9:10:00 am -- our accident train first appears onscreen, having crossed milepost 389.1, entering the S Coniston control point; he has a clear Green through the N and S Bolivar Control Points. Only having 1 block green at this point really isn't a problem ... the train itself is still 16 miles back from there.
9:15:00 am -- our accident train has gotten a little farther south, and the dispatcher has cleared another 11 miles of track, down to Wyvern.
9:35:00 am -- our accident train has moved onscreen, crossing the Bolivar control points. Notice that back at S Coniston, it's still red -- that's another train, following about 10 miles or so behind the accident train.
9:40:00 am -- moving out of Bolivar, and into the 11 mile stretch headed to Wyvern, the dispatcher hasn't yet lit up the track ahead ... note the 2nd train still back north of Bolivar.
9:45:00 am -- solidly into the stretch between Bolivar and Wyvern, the dispatcher has now cleared the accident train, as well as the train behind it. This is an excellent example of how dispatchers keep space between trains.
9:50:00 am -- the accident train is now cleared all the way to just short of the W&A. Had the accident not occurred, this train would have headed south on the W&A, and right on through Atlanta.
9:59:45 am -- the stage is set, as the accident train enters the 3 mile stretch between Wyvern and North Yard
10:02:00 am -- the accident train is just shy of North Yard at this point
10:03:30 am -- the front of accident train has crossed North Yard safely, and the derailment is about to occur. Note that on track 2, it's all lit up for a train to exit the Wyvern Yard. (note: the grey section is normal)
10:04:00 am -- uh, oh ... all of a sudden, the crossover switches throw a warning that the points are out of position (that's the blue boxes) ... note the previously cleared green signal dropped red at North Yard. This is a good example of the signal plant protecting a movement ... what was clear, is no longer due, to the malfunctioning switch point.
What actually happened, who knows ... we'll have to wait for the FRA report. What is certain is that the dynamics of the derailment popped the switch out of position (perhaps a car picked the switch?), and the switch plant detected and dutifully reported it, which is exactly what it's designed to do.
Now watch how fast events unfold :
10:04:22 am -- now we have a track occupied indication on trk 2 ... as you can see from the pictures in the opening comment, cars are stacked across both tracks ... so this indicates the derailment has progressed to the point where a car has shorted trk 2's detection circuit. Note the train coming out of Wyvern Yard who still has a Green at this point.
10:04:26 am -- the switches oddly enough no longer read in error ... I can only guess that the derailing railcars simply aren't blocking the switches at this point, or they are faking the circuit out by shorting the points to the rails. At any rate, note the train coming out of Wyvern yard now has a Red ... will he stop without running the Red?
10:04:40 am -- nope ...
10:04:49 am -- at this point, the derailment is pretty much over ... note the crossover switches are indicating position errors again. I'm sure the Wyvern Yard train's engineer right about now is squealing to a stop, and trying to get his heart under control ... he can't see the derailment over 3 miles ahead, but he for sure knows that Green signals don't suddenly drop Red for no reason ...
10:15:00 am -- stuff settles down and the trailing train coming down the Etowah Sub pulls in north of Wyvern. He isn't going anywhere for a while.
What is impressive about this whole sequence is that the entire derailment event lasts a mere 49 seconds.
